(1) Field of the Invention
This invention relates to an improvement in forging dies used for manufacture of a gear-shaped part made of a sheet metal, such as a pulley for a toothed belt.
(2) Description of the Prior Art
Gear-shaped parts made of sheet metal are widely used as pulleys for use under relatively low loads, such as pulleys for toothed belts (See, for instance, Japanese Patent Application Laid-Open (KOKAI) No. 61-238436 (1986)).
In the manufacture of a gear-shaped part such as a gear and a pulley for a toothed belt (the gear-shaped part will be hereinafter referred to simply as "the part"), for instance, there has been generally known the so-called spline forming process in which a flat steel sheet blank is press formed into a cup shape having a cylindrical peripheral wall portion, and the peripheral wall portion of the cup-shaped article (hereinafter referred to as "the work") is squeezed into a splined shape (See, for instance, Japanese Patent Application Laid-Open (KOKAI) No. 59-42144 (1984)).
According to the spline forming process, it is possible to obtain a predetermined strength required of the part, by appropriately setting the material and plate thickness of the blank, conditions of forming, etc. It is also possible to realize a marked reduction in weight of the part, as compared to that in a conventional manufacturing process in which a blank roughly formed by casting, forging or the like is finished to the final shape by mechanical working or the like. Application of the spline forming process to mass-production of a product has a great practical merit of low manufacturing cost because of the use of press working, which is higher in productivity than the conventional manufacturing process.
Therefore, particularly in the case of an automobile engine, transmission or the like incorporating a multiplicity of parts therein, manufacture of the parts by the spline forming process enables a reduction in the total weight of the engine or transmission and in the manufacturing cost.
More particularly, as shown in FIG. 1, the part 1 comprises a shallow closed-end cylinder (so-called cup-shaped body) with tooth portions 2 and bottom land portions 3 integrally formed at the outer periphery of the peripheral wall portion of the cylinder. The part 1 is obtained by forging, by a die and a punch, either a work 5 obtained by forming a positioning mount surface 4 at a central portion of a circular disklike blanked material, as shown in FIG. 2(a), or a work 6 obtained by drawing a peripheral portion of the work 5 to give a cylindrical shape, as shown in FIG. 2(b). FIGS. 3(a) and 3(b) illustrate the process of manufacturing the part 1 from the work 5 by forging, in which the work 5 is drawn in a cylindrical die 7 by a cylindrical punch 8, whereby the peripheral portion of the work 5 is drawn to give a cylindrical shape and, simultaneously, the peripheral wall portion 5a is formed with tooth portions 2 and bottom land portions 3. In FIGS. 3(a) and 3(b), numeral 9 denotes a counter punch, 10 a cushion pin, 11 a support plate, 12 a base plate, and 13 denotes a cushion pad.
The inner peripheral surface of the die 7 is provided with ridges 14 and grooves 15 for forming the tooth portions 2 and the bottom land portions 3 of the part 1, as shown in FIG. 4. Namely, the peripheral wall portion 5a of the work 5 is pressed by the ridges 14 to form the bottom land portions 3, and the peripheral wall portion 5a of the work 5 is forced into the grooves 15 to form the tooth portions 2.
According to, for instance, Japanese Patent Application Laid-Open (KOKAI) No. 59-42147 (1984), the shapes of the ridges 14 and the grooves 15 of the conventional die 7 have been as shown in FIG. 4. Therefore, the depths of the bottom land portions 3 are formed by taper surfaces 16 of the ridges 14, while the gaps between the tooth portions 2, or the side surfaces of the tooth portions 2, are formed by taper surfaces 17 of the ridges 14. Then, the heights of the tooth portions 2, or the crests of the tooth portions 2 are formed by taper surfaces 18 of the grooves 15. Thus, with the conventional die 7, the bottom land portions 3 and the side surfaces of the tooth portions 2 have been formed at a stroke, so that excessive tensile forces have been applied between the bottom wall portion 5b and the peripheral wall portion 5a of the work 5. Due to the excessive tensile forces, a rounded portion called die wear 19 might be generated at one end of the tooth portion 2 of the part 1, leading a reduction of the effective tooth length by l, as shown in FIG. 5. Or, particularly where the tooth height is set to be relatively large, as shown in FIG. 6, cracking might occur between the bottom wall portion 5b and the peripheral wall portion 5a of the work 5. FIG. 7 shows burrs 20 which are generated at an opening-side end portion of the peripheral wall portion 5a of the work 5.
To overcome the drawbacks of the die wear 19 and the like, there has been proposed a forging process as shown in FIGS. 8(a) and 8(b) (See Japanese Patent Application Laid-Open (KOKAI) No. 62-31770 (1987)). The forging process comprises a first step in which, as shown in FIG. 8(a), a work 6 preformed with a peripheral wall portion 6a is pushed into a first die 21, starting with a bottom wall portion 6b thereof, and then taken out of the first die 21. In a second step, as shown in FIG. 8(b), the work is pushed into a second die 22, starting with the opposite end thereof. According to the process, the peripheral wall portion 6a is squeezed in the opposite directions, so that the length of die wear is reduced. The forging process, however, is disadvantageous on a production efficiency basis, because the process is divided into the two steps and reversion of the work 6 and application of a lubricant to the surface of the work 6 must be performed two times each. In addition, the process has also a problem as to equipment cost due to the need to prepare two kinds of dies and punches.